The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. The oilfield services industry stimulates the productivity of hydrocarbon wells and injector wells in subterranean formations. The most common techniques use aqueous-based fluids to increase hydrocarbon productivity and include matrix acidizing, hydraulic fracturing, acid-fracturing, sand control, water-control, enhanced oil-recovery, and other techniques. A number of these techniques involve the injection of aqueous solutions of polymers that may be crosslinked and may render viscous gels.
In either hydraulic fracturing or water-control, viscosified polymer selection and modification is critical to achieve predictable fluid properties, specifically, fluid rheology, particulate-suspension in hydraulic fracturing, and gel strength in water-control. In these and other stimulation treatments, tailoring the interactions of the gel with either the formation or injected particulates that may comprise part of the treatment could benefit the overall outcome. For example, improved gel strength in water control applications could lead to longer treatment-lifetimes; efficient and predictable generation and subsequent breaking of gel viscosity can help to minimize damage in hydraulic fracturing.
In hydraulic fracturing fluids, a viscosified polymer solution is used to carry solids including proppant such as sand downhole and through the length of the fracture. The inability to carry proppant through the entire fracture length leads to the premature buildup of proppant in the tubulars, the presence of this buildup is referred to as a screenout. The occurrence of a screenout is highly undesirable because of the costs to remove the material. Screenouts most often occur when the viscosity of the fluid is insufficient under the bottomhole temperature, shear rates experienced in the tubulars and fracture, and pressure to carry the proppant throughout the length of the fracture. There is a need to improve the fluid rheology under downhole conditions and, as a result, extend the time for sand-settling to improve the success rate of hydraulic fracturing using crosslinked polymer solutions. A wide variety of polymers are used in the crosslinked-polymer fluids pumped in hydraulic fracturing, including polysaccharides, guar, derivatized guar (including hydroxypropyl guar, HPG, and carboxymethylhydroxypropyl guar, CMHPG), xanthan gum, and diutan. Crosslinking is usually performed by crosslinking the polymer hydroxyl groups using inorganic species such as zirconates, titanates, aluminates, and other species such as borates or protected derivatives of any of these crosslinkers.
The other key to a successful hydraulic fracturing treatment relates to the behaviour of the viscous fluid after proppant placement. The viscous, often crosslinked, carrier fluid carries proppant throughout the fracture. As fluid leakoff occurs through the fracture faces, the crosslinked fluid concentrates significantly in the proppant pack, often leading to significant gel damage in the proppant pack. This gel damage is often strongly adsorbed to the proppant itself and minimizes the expected productivity gains from the fracturing treatment. In order to decrease the fluid rheology after treatment and minimize polymer-gel damage in the proppant pack, “breaker chemicals” are often added either throughout the fracturing fluid or in encapsulated form to release into the fluid upon fracture closure. However, the methods by which these breaker additives are added into the fluid can lead to heterogeneous placement through the proppant pack which often leads to insufficient rheology-break throughout the fracture. For example, encapsulated breaker may be deposited only in selected areas of a proppant pack, not homogeneously. As a result, significant amounts of unbroken polymer gel damage may exist in packets throughout the fracture, specifically, packets of gel strongly adsorbed to the proppant.
Another example of a crosslinked polymer solution used in oilfield services is in water shutoff or water control, where solutions of polymer and crosslinker are injected downhole and, after a delay, form a rigid crosslinked gel under bottomhole conditions including exposure to high temperature and pressure. In water control applications, this rigid, crosslinked gel forms within the formation in the porous matrix. The viscosity of a water control solution should remain low during the injection downhole to minimize friction pressure or drag through the tubular and to ensure injectivity into the target formation. However, the critical parameters for success of a water control treatment are the rigidity or gel strength of the gel upon crosslink and minimized extrusion of the gel from the formation. In current water control formulations, there is a lack of a distinct covalent bond between the crosslinked polymer gel and the formation. This lack can lead to debonding of the gel from the formation and extrusion of the gel out of the porous matrix.
An effective way to treat proppant surfaces and surfaces of subterranean formations to encourage effective interactions with each other and with fluids possibly containing polymer is needed.